1. Field of the Invention
The present invention relates to a method for shaping a particular consolidated ceramic composite having relatively low strength and relatively high ductility.
2. Description of the Prior Art
Silicides of transition metals such as molybdenum disilicide and composites comprising matrices of the transition metal silicide reinforced with silicon carbide (SiC) are considered potential materials for structural applications due to their high melting points, excellent oxidation and corrosion resistance, low densities and good electrical and thermal conductivity. However, the use of MoSi.sub.2 is limited as a structural material due to its low ambient temperature, fracture toughness and poor elevated temperature strength.
The high melting point of MoSi.sub.2, coupled with its line compound characteristics, pose considerable difficulties in its processing. Consequently, powder processing has been the preferred fabrication route due to the lower processing temperatures that it affords. Unfortunately, it also results in the incorporation of silica (originally formed on the surfaces of the powders as an oxide layer) into the consolidated bodies. The presence of grain boundary silica in the consolidated bodies is detrimental to the high temperature properties of the MoSi.sub.2 composites since it softens above approximately 1,200.degree. C. and causes enhanced deformation of the MoSi.sub.2 body by possibly promoting viscous grain boundary sliding. Thus, control and reduction of the grain boundary silica is essential in order to improve the high temperature properties of MoSi.sub.2.
Prior art methods have endeavored to control the silica content (hence, the oxygen content), and thus improve the high temperature properties of MoSi.sub.2. For instance, Maxwell NACA RM E52B06 (1952)! varied the grain size and the carbon content and found that a fine grained material with carbon additions had better creep properties and a lower high temperature plasticity than a similar grain sized material without carbon additions. More recently, Maloy et al Journal of the American Ceramic Society, Vol. 74, page 2704 (1991)! also reported improved high temperature fracture toughness with increasing levels of carbon additions. However, substantial (.about.40%) weight losses were reported on consolidating these samples, resulting in uncontrolled formation of molybdenum rich second phases. Hardwick et al at Rockwell International Scripta Metallurgica et Materialia, Vol. 27, "Reaction Synthesis of MoSi.sub.2 from High Purity Elemental Powders," pages 391-394 (1992)! attempted to process oxygen-free MoSi.sub.2 by conducting all the powder handling and consolidation operations under a vacuum or inert atmosphere. However, these approaches Hardwick et al, supra; and Schwarz et al, Materials Science and Engineering, Vol. A155, page 75 (1992)! are impractical from the standpoint of processing bulk structural parts due to the difficulties involved in the scale-up of evacuation systems, as well as the excessive costs associated with such processes. Other methods of forming MoSi.sub.2 /SiC composites, as described in U.S. Pat. Nos. 4,927,792 and 5,000,896, still do not deal with the problem of controlling or limiting the oxygen content in the MoSi.sub.2 matrices.
In view of these considerations, it is clear that further enhancements in the properties of MoSi.sub.2 and MoSi.sub.2 /SiC composites are possible only by limiting/eliminating the silica in the matrix, along with close control of the overall stoichiometry of the body, through the use of economic processing schemes which do not necessitate elaborate care during powder handling. Such methods embodying these requirements are taught in copending U.S. patent application Ser. No. 08/538,004 filed Oct. 2, 1995, and U.S. Pat. Nos. 5,454,999 and 5,340,531, and represent considerable advancements in the art of processing of MoSi.sub.2 bodies. The entire contents and disclosures of U.S. patent application Ser. No. 08/538,004 and U.S. Pat. Nos. 5,454,999 and 5,340,531 are incorporated herein by reference.
Formation of shaped bodies of MoSi.sub.2 also poses significant challenges. Being hard and brittle at room temperature, MoSi.sub.2 thus has very poor machinability. Accordingly, room temperature shape forming is limited to such processes as grinding and polishing. In addition to being expensive and time consuming, such processes result in a considerable waste of expensive material. Such processing bottlenecks increase production costs, thereby hindering the large volume production of MoSi.sub.2 bodies.
A variety of conventional shape forming processes such as sintering, chemical vapor infiltration, liquid metal reactive infiltration and injection molding have been utilized in the fabrication of MoSi.sub.2 bodies. However, each of these processes has intrinsic limitations.
The use of sintering, for example, results in shrinkage of the body, the extent of which is dependent on the green density, sintering temperature, sintering atmosphere, etc. Moreover, recent studies on the sintering of MoSi.sub.2 bodies have shown that substantially high temperatures and very fine particle sizes are needed to achieve full densification by sintering, in addition to the use of hydrogen-containing atmospheres.
Chemical vapor infiltration Stinton et al, "Advanced Ceramics by CVD Techniques," American Ceramic Society Bulletin, Vol. 67, No. 2, pages 350-355 (1988); and Stinton et al, "Synthesis of Fiber Reinforced SiC Composite by CVI," American Ceramic Society Bulletin, Vol. 65, No. 2, pages 347-350 (1986)! of MoSi.sub.2 and SiC bodies, aside from needing appropriate gaseous precursors, is nevertheless limited by the very slow deposition rates 1-2 weeks!. Furthermore, the clogging of the exterior pore channels during the processing necessitates expensive modifications to the process such as the superimposition of appropriate temperature gradients and the use of intermediate (time-consuming and expensive) machining operations in order to form a pore-free, fully dense body.
Liquid metal infiltration, while being a well established, relatively quick and inexpensive method of forming dense MoSi.sub.2 /SiC bodies, has been traditionally plagued by the inability to achieve complete conversion of the reactant silicon and carbon; the result is the presence of residual silicon and carbon in the material, which is detrimental to the optical and mechanical properties. Furthermore, issues such as capillarity effects and wetting, which are dependent on the preform and melt characteristics, need considerable optimization as a prerequisite to extension of such techniques to systems of variable compositions. However, there have been recent successes in resolving this problem, as for example, by the methods of Chiang et al in U.S. Pat. No. 5,079,195 and Messner et al in J. Am. Ceram. Soc., Vol. 73, No. 5, "Liquid Phase Reaction Bonding of Silicon Carbide Using Alloyed Mo--Si Melts," pages 1193-1200 (1990). Similarly, injection molding, while certainly being amenable to the formation of complex shapes, is limited in the maximum dimensions of producible parts, due to the difficulties associated with the efficient burnout and removal of the organic binder material used in the green forming steps.
In view of the difficulties experienced in the traditional ceramic processing routes, traditional plastic deformation processes offer an attractive, worthwhile, low cost alternative to the net shape manufacture of MoSi.sub.2 articles in consideration of the ductile characteristics exhibited by MoSi.sub.2 above .about.1,200.degree. C. Plastic deformation offers the possibilities of a drastic reduction in the production costs compared to the earlier described processes while enabling larger scale production of bodies for use in structural applications.
However, the phenomenon of work hardening, wherein a body suffers increasing resistance to an applied deformation with increasing strain, makes shape-forming difficult to achieve in practice. It has been noted that the large grained MoSi.sub.2 materials undergo considerable work hardening at elevated temperatures. Such work hardening characteristics, while being welcome from a service consideration, are detrimental from a processing standpoint since they imply increasing resistance to deformation. The problem is further compounded considering the fact that the deformation processing of advanced materials is usually in the regime of 1,250.degree. C. and above, where the choice of die and plunger tooling materials becomes restricted and expensive with an increase in the operating stress levels of the forming presses. Furthermore, work hardening characteristics also increase the tendency of the body to fracture (crack) with increasing levels of deformation, thereby limiting the extent of deformation imparted in the forming operation. In addition, necking (plastic) instabilities may occur during the deformation process and limit the useful attainable deformation.
On the other hand, some metallic and ceramic materials undergo large deformations under relatively low stresses at temperatures above half their absolute melting temperatures, when their grain size is small, below 10 .mu.m. This phenomenon is referred to in the art as "superplasticity." For such metallic and ceramic materials, it is preferable to shape these materials by initially deriving materials comprising fine grain structures and subsequently deforming them under controlled conditions of strain rate and temperatures, thereby utilizing their favorable (superplastic) deformation characteristics. Such deformation occurs at stress levels far lower than the usual yield points (in coarse grained material) without the instabilities associated with necking. Thus, large, uniform, crack-free deformations of the order of several hundred percent can be obtained as a result of such deformation.
Low stress, high ductility-imparting deformation processes such as superplastic deformation have been used considerably in the past in the production of metallic/ceramic articles and composite articles possessing high strength at high temperature. In U.S. Pat. No. 3,519,503, Moore et al teach a forging process wherein certain alloys, including those suitable for high temperature gas turbine applications, may be shaped to complex configurations under very low pressures. The processes taught therein also depend on the proper processing of the alloys to their temporary condition of low strength and high ductility and maintain such conditions throughout the duration of the forming process. The "Gatorizing" process, as described in U.S. Pat. No. 3,519,503, is thus an excellent example of the utilization of the low flow stresses to produce hitherto unworkable superalloy forgings with very close tolerances.
An important point to be noted herein is that the design criteria for high temperature structural components, to which structural materials in general, and MoSi.sub.2 in particular have particular relevance, requires the use of alloys that maintain a high degree of strength at high temperature, in addition to oxidation resistance. Generally, while it has been possible to find materials which meet the high strength requirements at high temperatures, the very improvements in the service properties that are required of these materials are often achieved at the expense of the fabricability of the alloys. For instance, substantial difficulties, as encountered during the sintering of high strength borides and carbides, have been attributed to their covalent-bond characteristics which, while causing enhanced resistance to deformation, also restrict diffusion, so essential for the sintering operation. Fabrication of such bodies has thus been traditionally performed under conditions of high temperature and pressure, which increase the processing costs. Likewise, conventional casting routes, while generally providing the large grain sizes desirable for high temperature use, are unsuitable for high melting materials due to problems of crucible-melt interactions (in addition to lack of suitable containment materials) and the ubiquitous segregation problems. In addition, volatilization of certain elemental components is also frequently encountered while melting these materials. Powder routes, while seemingly offering a viable alternative for the production of large grain sizes, are also constrained by densification problems, especially for materials with coarse starting powder sizes. The primary reason is that the governing mechanisms for strengthening are the same as those of deformation in the service and processing temperature regimes. Such difficulties in processing seriously hinder the development of viable structural materials due to unfavorable economics involved in their processing.
In view of the potential attractiveness of the low stress, high ductility imparting deformation processes (superplastic deformation), it would be desirable to adapt this process to MoSi.sub.2 bodies. When subjected to uniaxial compressive stress at controlled strain rates and controlled temperatures, MoSi.sub.2 /SiC bodies show a high degree of uniform deformation at a constant stress. Furthermore, the stress levels associated with such deformation are in the range of 5-50 MPa, under reasonably high strain rates and at reasonably low temperatures. However, these low stress levels imply that the material is not best suited for use in load-bearing applications.
Fabrication to close tolerances is critical to the utility of a material on an economically feasible scale in aerospace engine applications. Many of the parts are moving/rotating, thus requiring close tolerances not only in initial assembly, but also during subsequent use at service temperatures. Therefore, the use of near net shape forming techniques to form parts to within tight dimensional tolerances would greatly enhance the economic and technical viability of MoSi.sub.2, provided that the products would be useful in load-bearing applications.
It is an object of the present invention to provide a method of producing substantially silica-free compositions of matter comprising the matrix of molybdenum disilicide having SiC dispersed therein.
It is another object of the present invention to produce articles of high formability (temporary high ductility and low strength condition) through providing a microstructure that is substantially homogeneous, both in composition and structure, which possesses fine equiaxed grains of MoSi.sub.2 with a fine, uniform dispersion of SiC therein, and with substantially low silica content.
It is a further object of the present invention to shape-form the bodies to the desired configuration and tolerances in this temporary condition of high formability.
An additional object of the present invention is to retrieve the bodies from their temporary condition of low strength and high ductility to one of high strength and low ductility and, most importantly, to control the level of such achievable strength maximum through the control of the starting alloy composition.
Yet another object of the present invention is to indirectly achieve such control through a control of the maximum grain size obtainable after a heat treatment step.
A further object of the present invention is to control such attainment of grain size through control of the volume of SiC in the matrix which, in turn, is obtainable through appropriate control of the starting composition.
A still further object of the present invention is to provide for improved reliability of the objects manufactured.
It is an additional object of the present invention to produce intricate-shaped articles to close tolerances with minimum wastage of charge material and with a minimum of post-finishing operations such as grinding, etc.
Finally, it is yet another object of the present invention to provide articles containing MoSi.sub.2 and SiC that are useful at elevated temperature or at ambient temperature through appropriate modifications in the processing conditions such as composition, grain size and the like.